1. Field of the Invention
The present invention generally relates to a thin film magnetic memory device. More particularly, the present invention relates to a random access memory (RAM) including memory cells having a magnetic tunnel junction (MTJ), and an information programming method.
2. Description of the Background Art
An MRAM Magnetic Random Access Memory) device has attracted attention as a memory device capable of non-volatile data storage with low power consumption. The MRAM device is a memory device capable of non-volatile data storage using a plurality of thin film magnetic elements formed in a semiconductor integrated circuit and also capable of random access to each thin film magnetic element.
In particular, recent announcement shows that the use of thin film magnetic elements having a magnetic tunnel junction (MTJ) as memory cells significantly improves performance of the MRAM device. The MRAM device including memory cells having a magnetic tunnel junction is disclosed in technical documents such as “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC Digest of Technical Papers, TA7.2, February 2000, “Nonvolatile RAM based on Magnetic Tunnel Junction Elements”, ISSCC Digest of Technical Papers, TA7.3, February 2000, and “A 256 kb 3.0V 1T1MTJ Nonvolatile Magnetoresistive RAM”, ISSCC Digest of Technical Papers, TA7.6, February 2001.
FIG. 31 schematically shows the structure of a memory cell having a magnetic tunnel junction (hereinafter, sometimes simply referred to as “MTJ memory cell”).
Referring to FIG. 31, the MTJ memory cell includes a tunneling magneto-resistance element TMR having an electric resistance varying according to the storage data level, and an access element ATR for forming a path of a sense current Is flowing through tunneling magneto-resistance element TMR in data read operation. Since a field effect transistor is typically used as access element ATR, access element ATR is hereinafter sometimes referred to as access transistor ATR. Access transistor ATR is coupled between tunneling magneto-resistance element TMR and a fixed voltage (ground voltage Vss).
A write word line WWL for data write operation, a read word line RWL for data read operation, and a bit line BL are provided for the MTJ memory cell. Bit line BL is a data line for transmitting an electric signal corresponding to the storage data level in data read operation and data write operation.
FIG. 32 is a conceptual diagram illustrating data read operation from the MTJ memory cell.
Referring to FIG. 32, tunneling magneto-resistance element TMR has a ferromagnetic material layer FL having a fixed magnetization direction (hereinafter, sometimes simply referred to as “fixed magnetic layer”), and a ferromagnetic material layer VL that is magnetized in the direction according to an external magnetic field (hereinafter, sometimes simply referred to as “free magnetic layer”). A tunneling barrier (tunneling film) TB is interposed between fixed magnetic layer FL and free magnetic layer VL. Tunneling barrier TB is formed from an insulator film. Free magnetic layer VL is magnetized either in the same direction as, or in the opposite direction to, that of fixed magnetic layer FL according to the storage data level. Fixed magnetic layer FL, tunneling barrier TB and free magnetic layer VL form a magnetic tunnel junction.
In data read operation, access transistor ATR is turned ON in response to activation of read word line RWL. This allows a sense current Is to flow through a current path formed by bit line BL, tunneling magneto-resistance element TMR, access transistor ATR and ground voltage Vss.
The electric resistance of tunneling magneto-resistance element TMR varies according to the relation between the magnetization directions of fixed magnetic layer FL and free magnetic layer VL. More specifically, when fixed magnetic layer FL and free magnetic layer VL have the same (parallel) magnetization direction, tunneling magneto-resistance element TMR has a smaller electric resistance than when they have opposite (antiparallel) magnetization directions.
Accordingly, when free magnetic layer VL is magnetized in one of the above two directions according to the storage data level, a voltage change produced in tunneling magneto-resistance element TMR by sense current Is varies depending on the storage data level. Therefore, by precharging bit lines BL to a prescribed voltage and then applying sense current Is to tunneling magneto-resistance element TMR, the storage data of the MTJ memory cell can be read by sensing the voltage on bit line BL.
FIG. 33 is a conceptual diagram illustrating data write operation to the MTJ memory cell.
Referring to FIG. 33, in data write operation, read word line RWL is inactivated and access transistor ATR is turned OFF. In this state, a data write current is applied to write word line WWL and bit line BL in order to magnetize free magnetic layer VL in the direction according to the write data level. The magnetization direction of free magnetic layer VL is determined by the directions of the data write currents flowing through write word line WWL and bit line BL.
FIG. 34 is a conceptual diagram illustrating the relation between the data write current and the magnetization direction of tunneling magneto-resistance element TMR in data write operation to the MTJ memory cell.
Referring to FIG. 34, the abscissa H(EA) indicates a magnetic field that is applied to free magnetic layer VL of tunneling magneto-resistance element TMR in the easy-axis (EA) direction. The ordinate H(HA) indicates a magnetic field that is applied to free magnetic layer VL in the hard-axis (HA) direction. Magnetic fields H(EA), H(HA) respectively correspond to two magnetic fields generated by the currents flowing through bit line BL and write word line WWL.
In the MTJ memory cell, fixed magnetic layer FL is magnetized in the fixed direction along the easy axis of free magnetic layer VL. Free magnetic layer VL is magnetized either in the direction parallel (the same as) or antiparallel (opposite) to that of fixed magnetic layer FL along the easy axis according to the storage data level (“1” and “0”). Hereinafter, Rmax and Rmin (where Rmax>Rmin) denote the electric resistances of tunneling magneto-resistance element TMR corresponding to the two magnetization directions of free magnetic layer VL. The MTJ memory cell is thus capable of storing one-bit data (“1” and “0”) according to the two magnetization directions of free magnetic layer VL.
The magnetization direction of free magnetic layer VL can be rewritten only when the sum of the applied magnetic fields H(EA) and H(HA) reaches the region outside the asteroid characteristic line in FIG. 34. In other words, the magnetization direction of free magnetic layer VL will not change if an applied data write magnetic field corresponds to the region inside the asteroid characteristic line.
As shown by the asteroid characteristic line, applying a magnetic field of the hard-axis direction to free magnetic layer VL reduces a magnetization threshold value required to change the magnetization direction along the easy axis.
When the write operation point is designed as in the example of FIG. 34, a data write magnetic field of the easy-axis direction is designed to have strength HWR in the MTJ memory cell to be written. In other words, a data write current to be applied to bit line BL or write word line WWL is designed to generate a data write magnetic field HWR. Data write magnetic field HWR is commonly defined by the sum of a switching magnetic field HSW required to switch the magnetization direction and a margin ΔH. Data write magnetic field HWR is thus defined by HWR=HSW+ΔH.
In order to rewrite the storage data of the MTJ memory cell, that is, the magnetization direction of tunneling magneto-resistance element TMR, a data write current of at least a prescribed level must be applied to both write word line WWL and bit line BL. Free magnetic layer VL in tunneling magneto-resistance element TMR is thus magnetized in the direction parallel (the same as) or antiparallel (opposite) to that of fixed magnetic layer FL according to the direction of the data write magnetic field along the easy axis (EA). The magnetization direction written to tunneling magneto-resistance element TMR, i.e., the storage data of the MTJ memory cell, is held in a non-volatile manner until another data write operation is conducted.
A memory device commonly conducts normal operation such as data read operation and data write operation based on program information stored therein in a non-volatile manner. Typically, information for use in control of the redundant structure (the structure for replacing a defective memory cell with a spare memory cell) is stored as program information. In the redundant structure, at least defective addresses for specifying defective memory cells must be stored as program information.
In the conventional memory device, program information is programmed by blowing fuse elements with laser or the like. However, this requires special equipments such as a trimming device for laser blowing, thereby increasing the time and costs required for the programming operation.
Such programming operation is conducted at wafer level. Therefore, if defective memory cells are detected at wafer level and defective addresses corresponding to the detected defective memory cells are programmed in the memory device before packaging process, it is difficult to handle defects generated after the packaging process, resulting in reduction in yield.
The above MTJ memory cells are capable of non-volatile data storage. Therefore, in the MRAM device, required information may be programmed with magnetic memory elements that are the same as, or similar to, the MTJ memory cells used as normal memory cells.
However, this structure requires frequent reset operation unless the initial state and program state of the magnetic memory elements used for the programming operation are clearly defined. This may possibly hinder high-speed operation of the memory device. Moreover, a current must be supplied to the magnetic memory elements in order to read the programmed information therefrom. Therefore, regarding operation of reading the program information, the program elements must have as high operation reliability as the normal memory cells.